1. Field of the Invention
The present invention relates to an optical recording and reproducing apparatus and method, and more particularly, to an optical recording and reproducing apparatus and method in which in playing different types of recording media to record or reproduce data into or from the media, a light of a first wavelength and a light of a second wavelength are used for one and another, respectively, of the recording media, whereby a compact design of such an apparatus can be attained.
2. Description of Related Art
Storage media, into or from which data is optically written or read, include optical discs such as CD (compact disc; a registered trademark), CD-ROM, CD-R, etc. In addition, other new types of recording media, including a DVD (Digital Versatile disc) capable of storing a large volume of data, have recently been developed and also under development.
To read digital data from a storage media like an optical disc, a laser light is focused on the media, and a reflected part of the laser light from the media is detected and converted to a binary data.
For data write or read into or from such a high-density optical disc, a laser light having a short wavelength (.lambda.) (for example, .lambda.=780 nm for playback of a CD and .lambda.=635 to 650 nm for replaying a DVD on which data are recorded at a higher density) is used and focused to a narrower area on the disc through an objective or an objective lens having a large numerical aperture (NA) (for example, NA=0.45 for playback of a CD and NA=0.6 for replaying a DVD), and a reflected part of the laser light from the disc is detected to reproduce a desired recorded data from the reflected laser light.
If an objective lens having such a large NA is used for the above purpose, a skew of the optical disc will increase an aberration in the reflected light. To reduce such an aberration, DVD substrate is made of a laser light-transmissive material and designed thicker (0.6 mm) than CD substrate (1.2 mm).
The focused spot size varies as a function of the values of an objective lens NA and laser light wavelength .lambda., namely, it is proportional to a .lambda./NA ratio), and the magnitude of a spherical aberration in a reflected light depends upon the thickness of a substrate used in the disc. Therefore, an optical system capable of reading data from a conventional CD is not compatible with a DVD. On the contrary, an optical system for playback of DVD cannot compatibly be used to read data from CD.
In these situations, however, it is predictably believed that the conventional optical discs such as CD will be coexistent with high-density optical discs such as DVD. For playback of such optical discs, it is extremely inconvenient to use a dedicated player for each of such different types of optical discs.
To avoid the above inconvenience, some methods have been proposed in which a single apparatus is used to play back a plurality of optical discs different in recording density and substrate thickness from each other.
A typical one of such solutions is known from the disclosure in the U.S. Pat. No. 5,446,565 in which an objective lens and a holographic optical element (HOE) is used in combination. FIG. 1 schematically shows the principle of the technology disclosed in the official gazette of the above United States Patent. As shown, a laser light having a wavelength of, for example, 650 nm, is irradiated to a CD 103 or DVD 104 through an HOE 101 and objective lens 102. As will be seen from FIG. 2, the HOE 101 has formed thereon a blazed or four-step hologram consisting of concentric annular zones (wave crests and troughs in sectional form). Because of the hologram made of the annular zones, an incoming light of 650 nm in wavelength is divided into a zero-order diffracted light (transmitted directly through the HOE 101) and a first-order diffracted light as shown in FIG. 3. The zero-order diffracted light is used for CD, while the first-order diffracted light is for DVD. The HOE 101 is optimized to substantially annul the diffracted lights of other orders.
The objective lens 102 is optimized for playback of the DVD 104. Thus, the zero-order diffracted light having passed through the HOE 101 is focused by the objective lens 102 onto a data recording layer of the DVD 104 having a 0.6 mm-thick substrate, as shown in FIG. 1. The pitch of the annular zones on the HOE 101 is optimized so that when the first-order diffracted light is focused through the objective lens 102 onto the CD 103 having a substrate of 1.2 mm in thickness, a spherical aberration due to a difference in thickness from the DVD 104 can be corrected. Also the annular zones have smaller diameters than the effective diameter of the objective lens 102 for an optimum NA for the CD 103. Thus, the first-order diffracted light having passed through the objective lens 102 is focused onto the data recording layer to the limit of diffraction to define a good light spot.
Furthermore, since the annular zone pitch of the HOE 101 is optimized for the light spot on the CD 103 to be a few hundreds microns from the light spot on the DVD 104 along the optical axis, each of the light spots will not affect a reproduced RF signal from the other light spot.
However, as the laser light having a wavelength as short as 650 nm is used in an optical pick-up of the above-mentioned type, data can be reproduced from a normal CD, but not from CD-R. CD-R is designed for both data read and write. Also it is designed to reflect a light having a wavelength falling within a bandwidth of 780 nm, and will thus absorb almost all of a light of 650 nm in wavelength used in the DVD player. This is why the above-mentioned type of optical pick-up cannot reproduce data from CD-R.
For solution of the above problems, the applicant of the present invention proposed an optical pick-up capable of replaying both CDs including CD-R and DVDs, as disclosed in the Japanese Patent Application No. 8-121337. FIGS. 4 and 5 show such pick-up devices, respectively. FIG. 4 shows an optics used in the optical pick-up for playback of the DVD 104, while FIG. 5 shows an optics for playback of the CD 103.
For playback of the DVD 104, a light source 111B generating a laser light of 780 nm in wavelength is turned off while a light source 111A generating a laser light of 650 nm is turned on, as shown in FIG. 4. A laser light emerged from the source 111A is spilt by a grating 112A into substantially three laser lights. These laser lights are passed through a dichroic prism (DP) 113 and polarizing beam splitter (PBS) 114, and incident upon a collimator lens 115. The collimator lens 115 is provided to convert the incident divergent lights to parallel ones which will be incident upon an HOE 117 through a .lambda./4 plate 116. After the HOE 117, there is provided a refractive objective lens 118 designed optimal for the DVD 104. Therefore, the HOE 117 is optimized by the refractive objective lens 118 to correct a spherical aberration in a light having a wavelength of 780 nm focused onto the CD 103, and thus it will not substantially work with a light of 650 nm in wavelength.
As shown as enlarged in scale in FIG. 6, the HOE 117 allows almost all the laser light having a wavelength of 650 nm to pass through it. Namely, a zero-order diffracted light goes out of the HOE 117. This laser light is focused by the refractive objective lens 118 onto a data recording layer of the DVD 104 having a 0.6 mm-thick substrate. Since the refractive objective lens 118 is optimized for no spherical aberration to occur when the laser light is incident upon the DVD 104, a light spot having been collimated to the limit of diffraction will be focused on the DVD 104.
On the contrary, for playback of the CD 103, the source 111A of a laser light of 650 nm in wavelength is turned off while the source 111B for the laser light having a 780-nm wavelength is turned on, as shown in FIG. 5. The laser light is incident onto a dichroic prism 113 through a grating 112B. The dichroic prism 113 allows the laser light of 650 nm in wavelength to pass through it while reflecting a laser light having the wavelength of 780 nm. Thus, the laser light reflected by the dichroic prism 113 is incident upon the HOE 117 through a polarizing beam splitter 114, collimator lens 115, and a .lambda./4 plate 116.
The annular zone pitch of the HOE 117 is designed optimum to correct a spherical aberration caused by a difference in substrate thickness between the DVD 104 and CD 103 when a first-order diffracted light of 780 nm in wavelength and the refractive objective lens 118 are used in combination, as shown in FIG. 7. Also, the diffracted light on the HOE 117 is defined only in an area smaller than the pupil diameter of the refractive objective lens 118 for the DVD 104 to agree with the NA of the CD 103. Thus, the laser light of 780 nm in wavelength is irradiated to define on the data recording layer of the CD 103 a focused spot having been collimated to the limit of diffraction. Thus, a stable data reproduction is possible with little stray light and deteriorated efficiency for light utilization.
The laser light reflected from the CD 103 or DVD 104 is incident upon the polarizing beam splitter 114 through the refractive objective lens 118, HOE 117, .lambda./4 plate 116 and collimator lens 115. The return light from the optical disc has a polarized plane 90 deg. turned from the incident light upon the optical disc since it has passed through the .lambda./4 plate 116 twice. Thus, the return light is reflected by the polarizing beam splitter 114 and incident upon a photodiode (PD) 120 through a multiple lens 119. Date recorded on the optical disc can be reproduced from an output from the photodiode 120.
In the prior art disclosed in the Japanese Patent Application No. 8-121337, however, as the two laser sources 111A and 111B for laser lights of different wavelengths are disposed on the optical axis of the refractive objective lens 118, the dichroic prism 113 is required to divide the optical axis to about 90 deg., which will increase the number of necessary parts and manufacturing costs as well as in a larger structure of the apparatus.